In the last few decades birders and biologists alike have noticed that spring migration is changing. Species are arriving on their breeding grounds earlier each year. It’s clear there’s a link between climate change and shifting travel dates, but a new study reveals that individual black-tailed godwits are very consistent in their migratory timing, challenging assumptions about how warmer weather shifts behavior.

These leggy, reddish shorebirds winter in Spain and Portugal. They return to their Icelandic breeding grounds each spring, between mid-May and mid-April, often nesting a month after arrival. A team of biologists who closely track their movements have noted that the birds arrive two weeks earlier today than they did twenty years ago.

To figure out why, they first tackled the long-held assumption that balmier weather might trigger individual birds to take flight sooner each season. Poring over 14 years of records for 54 individual godwits, they discovered something curious: Each bird returned year after year on roughly the same day.

What did change was the birds’ nesting date. Females laid their eggs sooner after their arrival than their parents had, which of course meant that their offspring hatched earlier. Thus, the authors propose, fledglings left their breeding grounds earlier than previous generations, to return earlier in the subsequent spring. So the change in arrival time is due to each new generation making the journey on slightly earlier dates than the previous, they report in the Proceedings of the Royal Society B.

“It’s a particularly important question because the species which are not migrating earlier are declining in numbers,” the study’s lead author, Jenny Gill of the University of East Anglia, said in a press release.

Birds with the most dramatic advances in migration are those who—like this godwit subspecies—travel short distances each year, the authors note. Long-distance migrants fare worse because by the time they arrive in spring, they have a smaller window for breeding and nesting, meaning their offspring will hatch at roughly the same time annually. Given the many risks shorebirds face, from sea-level rise to development, the godwit’s ability to adapt on the fly may be more important than ever.

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